Challenging Bollworm Year Expected In The Delta BY JEFF GORE, ANUGUS CATCHOT, DON COOK AND WHITNEY CROW
2019 Sprays Delayed In a more typical year, growers can make a spray when the egg numbers increase and peak, which usually occurs about the middle of July. The egg lay drops to almost zero for a period of time and then peaks again in about a month, triggering a second spray. In 2019, we never really saw the first peak in eggs that makes the decision to spray easy. In cotton, we typically got reports of low egg levels that ended up turning into low levels of larvae and damage that got worse over time, especially north of Highway 82. As a result, we had a lot of sprays go out a little later than optimum in two-gene cotton varieties. In most cases, even if the sprays went out late, the survival and damage levels we saw were mostly not economical. In other words, the sprays worked, just not as good as they could have or as good as we would have liked. Extended Corn Planting Hurt The population dynamics of bollworms in cotton and soybeans are mostly influenced by corn. We believe the extended planting window and
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COTTON FARMING | MAY 2020
Follow thresholds when treating for bollworms.
COURTESY OF MISSISSIPPI STATE UNIVERSITY
T
he 2019 season was one of the most challenging years we have had from a bollworm standpoint in both cotton and soybeans. The issue was not from a numbers position where they overwhelmed the traits and foliar insecticides. It was more an issue of population dynamics and how the egg lays occurred. Rather than a normal population growth curve where the number of eggs starts off low and increases over a seven-day period ending with a heavy egg lay, we had a low trickling of eggs that persisted over several weeks. This made decisions to spray very difficult.
later planting dates for corn were a primary cause for what we experienced in 2019. Trying to predict what insect populations will be like during July and August is almost impossible in April. And this year is setting up a lot like last year from a bollworm standpoint. Getting corn planted in a timely manner has been difficult with the rains this spring and will likely influence bollworms in cotton and soybeans. As of the third week in April, we expect another challenging year from a bollworm perspective. We will collect bollworms from crimson clover in a couple weeks, but that will not tell us a lot about the upcoming crop season. Because corn is such a good host during June, even low populations during the spring can turn into big populations during July. Similarly, big populations in clover don’t really matter if green corn silks
are not available when bollworms that developed on clover emerge as adults. The point is that corn is the key. Follow Thresholds For 2020, the best advice we can give is be prepared for anything and follow our thresholds. In twogene cotton (Bollgard II, TwinLink, WideStrike), we are still suggesting an egg threshold of 20% of plants with at least one egg. In three-gene cotton (Bollgard 3, WideStrike 3 and TwinLink Plus), we are sticking with a damage threshold. We will monitor what is happening in corn and at more southern locations to give updates throughout the summer. Drs. Gore, Catchot, Cook and Crow all work at Mississippi State University’s Delta Research and Extension Center in Stoneville. COTTONFARMING.COM
